Steam valves are generally installed on an upstream side of a steam turbine, in order to control/shut off a flow rate of a steam. Steam valves installed on an upstream side of a high pressure turbine are referred to as a main-steam stop valve and a steam regulating valve, which are serially disposed. Steam valves installed on an upstream side of an intermediate pressure turbine are referred to as a combination reheat steam valve (hereinafter also referred to as “reheat steam valve”) having a casing in which an intercept valve and a reheat steam stop valve are combined (see, for example, Review of Thermal Power Generation, edited by Toru SEMA, Institute of Electrical Engineers, First Edition published on Oct. 25, 2002 (page 143, FIGS. 6.38 and 6.39)).
As shown in FIG. 16, supplied through a steam inlet pipe 14 to such a reheat steam valve 1 is a steam which has been overheated by a reheater of a boiler disposed on an upstream side. The steam having passed through the reheat steam valve 1 is introduced to an intermediate pressure turbine disposed on a downstream side through a steam outlet pipe 15.
As shown in FIG. 16, the reheat steam valve 1 includes: an intercept valve 20 that is driven from above to move in an up and down direction; a reheat steam stop valve 25 connected to a lower side of the intercept valve 20, the reheat steam stop valve 25 being driven from below to move in the up and down direction; and a cylindrical strainer 6 that is disposed to surround the intercept valve 20 and the reheat steam stop valve 25, the strainer 6 preventing foreign matters from the upstream boiler from being mixed into the downstream intermediate pressure turbine. The strainer 6 is formed of a porous plate.
The intercept valve 20 has: a valve rod 21 that is driven from above to move in the up and down direction; and a valve body 22 annularly disposed on the valve rod 21, the valve body 22 having a recess 22a in a lower surface thereof.
The reheat steam stop valve 25 has: a valve rod 26 that is driven from below to move in the up and down direction; and a valve body 27 disposed on the valve rod 26 to project therefrom in a circumferentially outer direction, the valve body 27 being capable of being received in the recess 22a of the valve body 22 of the intercept valve 20.
Disposed below the intercept valve 20 and the reheat steam stop valve 25 is a valve seat 9 capable of contacting the valve body 22 of the intercept valve 20 and the valve body 27 of the reheat steam stop valve 25. When the intercept valve 20, the reheat steam stop valve 25, or both, are in press-contact with the valve seat 9, a steam flow path can be closed.
As shown in FIG. 17, since the steam outlet pipe 15, which is bent at an acute angle, is disposed on the downstream side of the reheat steam valve 1, a primary flow of the steam generates a secondary flow. Because of another steam flow flowing into a part of the secondary flow where a speed thereof is slow, intensive secondary flows overlap each other in a steam valve outlet flow, which finally invites an increase in damage in pressure loss.
More specifically, in FIG. 17, after a steam primary flow F having flown into the reheat steam valve 1 passes through the strainer 6, the steam primary flow F passes through a gap between the valve body 22 of the intercept valve 20 and the valve body 27 of the reheat steam stop valve 25, and the valve seat 9. Thereafter, the steam reaches an inside of the steam outlet pipe 15. At this time, since a space into which the steam flows is abruptly widened, a part of a flow F3 becomes a secondary flow swirl, causing a loss.
In addition, another part of flow F1, which cannot follow an abrupt turning of the steam when the steam goes out to a steam outlet flow path 32, is pressed onto a lower portion of the steam outlet pipe 15, and is then discharged as a primary flow. On the other hand, in an upper portion of the steam outlet pipe 15, there is generated a secondary flow F2 which is then discharged while forming a swirl in the upper portion of the steam outlet pipe 15.
At this time, the primary flow F1 in the lower portion of the steam outlet pipe 15 and the secondary flow F2 in the upper portion of the steam outlet pipe 15 would strongly collide with each other, to thereby generate a serious pressure loss. FIG. 18 is a view showing a flow of a steam when viewed from the outlet side (viewed along an arrow X in FIG. 17). As shown in FIG. 18, the primary flow F1 flows through a range S and is discharged, while the secondary flow F2 is discharged while turning in a direction of an arrow A.
In addition, a steam passing through a surface of the strainer 6 passes through holes in the strainer 6. Thus, an axial component of velocity of a cylinder of the strainer 6 is induced to generate a strong secondary flow.
In general, it is said that, when a pressure loss of a steam valve is reduced by 1%, a heat rate of an overall steam turbine is improved by 0.1% (see, Turbo Machine, Vol. 30, No. 7), and thus a further reduction of pressure loss in the steam valve is an important object that should not be unnoticed.